Piezoelectric resonators may be used in various components such as oscillators and filters over a wide range of frequency applications. These resonators may also be employed in generating clock signals in integrated circuits, where the frequency of vibration is directly related to a clock frequency.
Piezoelectric resonators may be thought of as solid state transducers which can convert mechanical energy into electrical energy and electrical energy back into mechanical energy, depending upon how the resonators are configured. The mechanical energy manifests itself as vibrations within the piezoelectric material of the resonator.
FIGS. 1A and 1B show different modes of resonance in conventional piezoelectric resonators. The various modes of resonance may be defined relative to the geometry of the piezoelectric material producing the vibrations. In FIG. 1A, piezoelectric resonator 100 may include a piezoelectric substrate 115 having a first electrode 105 coupled to its upper surface, and a second electrode 110 coupled to its lower surface. When an electric excitation signal is applied to the electrodes 105 and 110, an electric field 120 may be induced within the piezoelectric substrate 115. The electric field 120 may cause a width vibrational mode 125, where the frequency of vibration may depend upon the width of the piezoelectric substrate (W). The width vibrational mode may also be referred to as a “d31” vibration, where d31 is a piezoelectric coefficient related to the width dimension (i.e., lateral dimension as shown in FIG. 1A) of the piezoelectric substrate 115.
As shown in FIG. 1B, the same electric field 120 may also cause a thickness vibrational mode 130 in the piezoelectric substrate 115. Here the frequency of vibration may depend upon the thickness of the piezoelectric substrate (T). The thickness vibrational mode may also be referred to as a “d33” vibration, where d33 is a piezoelectric coefficient related to the thickness dimension (i.e., vertical dimension as shown in FIG. 1B) of the piezoelectric substrate 115.
A coefficient of electromechanical coupling, denoted by kt2, represents the efficiency of energy conversion, such that a higher coefficient of electromechanical coupling indicates that mechanical energy is more efficiently converted to electrical energy.
In practice, thickness vibrational modes are commonly exploited in piezoelectric resonators because this mode tends to exhibit a high coefficient of electromechanical coupling, kt2. However, resonators utilizing thickness-only vibrational modes suffer from the drawback that T may not provide the freedom to define the resonant frequency by layout design, which is an advantage of resonators having width-only vibrational modes. It should be noted that the layout design is a pre-fabrication process, which cannot be altered after fabrication. In other words, by doing only one fabrication run, the width vibrational modes may provide multiple frequencies of operation on a single wafer, but the thickness vibrational mode resonators only provide one frequency.
On the other hand, width vibrational mode piezoelectric resonators having substrates made from Aluminum Nitride exhibit a coefficient of electromechanical coupling that is nearly one third the value associated with thickness vibrational mode resonators. This means that efficiency of width vibration mode resonators may be low. However, the width may be easily alterable during a pre-fabrication design process by coupling multiple vibrational resonators to form a multi-finger resonator. The multiple fingers may be adjacently placed and mechanically coupled by their edges so that the entire structure, including multiple fingers, vibrates as a single body. The multiple fingers may be electrically connected in parallel so that the entire structure is electrically equivalent to a single resonator.
There are currently no piezoelectric resonators which can take advantage of the positive attributes of both the width and thickness vibrational modes. Accordingly, there is a need for piezoelectric resonators which can combine the advantages of width and thickness vibrational modes to improve efficiency, while still preserving the single-chip (i.e., single-fabrication) multiple frequency capability.